1. Field of the Invention
The invention pertains to acoustical well logging and more particularly to digitally treating well-logging signals so as to more accurately determine the presence, location and amplitude of the compression wave and the "shear" wave in a more reliable manner than in the prior art.
2. Description of the Prior Art
Conventionally, the received analog logging signal is detected at the surface from the cable connected to the downhole logging tool and amplified in a manually controlled amplifier. The output of the amplifier is applied to time measuring circuits where the zero-crossing location of the signal after the arrival of a signal greater than a preset threshold level, is used as the time reference point for the onset of the compression wave. As mentioned, it is conventional that the gain of the amplifier be log-operator dependent. Further, it is common to make one logging run while setting the amplifier and then make another run at the amplifier setting for developing the actual logging signal to be utilized in analysis.
Because of the presence of noise it is also often desirable to use correlation techniques in order to ascertain the arrival time of the "shear" wave. The use of analog correlation techniques are limited by the dynamic range of the correlation devices.
Therefore, it is desirable to use digitization, or digitized data, for correlation purposes. Fast sample rates on the order of two microseconds are often needed. The desirable dynamic range of 60-70 d.b. dictates the use of approximately twelve bits in the analog-to-digital converter. ADC circuits and amplifiers operating under these conditions are expensive and only marginally reliable. Further, when the processing is done in real time, the amplifier gain must be preset prior to the arrival of the acoustic wave to be analyzed. Since using a prior received signal may not be the same as for a later signal, such setting is undesirable.
Therefore, it is a feature of the present invention to provide an improved method and apparatus for digitally sampling an acoustical logging signal and processing such signal at a delayed time or in an expanded time frame.
It is another feature of the present invention to provide an improved method and apparatus for establishing an automatic gain control value from a measured value of the logging signal prior to the arrival of the compression wave, digitizing a portion of the logging signal prior to amplification and controlling the gain of the digital amplifier with the automatic gain control value, thereby making the log amplifier independent of a manual setting by the log operator.
It is yet another feature of the present invention to provide an improved method and apparatus wherein the logging signal is subjected to peak detection having a floating threshold based on an rms determination of residual noise, such peak detection determining the approximate time position and approximate appearance of the compression wave for convolving against the digitized logging signal for accurate time placement of the compression wave.